Adsorptive membranes for trapping viruses

ABSTRACT

A disposable, virus-trapping membrane, and a corresponding method to remove viruses from solution are described. The membrane includes a disposable, micro-porous filter membrane and a ligand immobilized on the membrane. The ligand irreversibly and selectively binds viruses. The ligand also has a pKa sufficiently high to repel antibodies via electrostatic charge repulsion.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.11/776,774, filed Jul. 12, 2007, which claims priority to provisionalapplication Ser. No. 60/830,917, filed Jul. 14, 2006, both of which areincorporated herein by reference.

BACKGROUND

Viral clearance is essential for manufacturing safe,biotechnology-derived pharmaceuticals such as monoclonal antibodies(mAbs), recombinant proteins, fusion proteins, sera and media, and thelike. Regulatory agencies worldwide mandate removal of viralcontaminants from a host of products headed into commercial markets. Thenanometer-scale of viruses complicates their separation frombiopharmaceutical intermediates because the viral particles (due totheir size) bind only to the surface of chromatography beads. Virusparticles are too large to enter the pores of conventionalchromatographic beads. Thus, the binding capacity of conventionalchromatographic beads for viruses is much smaller than it is forimpurities that can enter and bind within the pores.

Chromatographic beads themselves are a relatively expensive commercialproduct. To operate at peak levels, the beads must have a very nearlymonodisperse particle size, in combination with a tightly-controlledpore size. As a consequence, chromatographic beads are designed forregeneration so that they can be re-used over many purification cycles(to keep manufacturing costs down). While this approach does, in fact,keep material costs in check, it is not without drawback. Most notably,when the beads are to be recycled, the virus-ligand binding must bereversible. In short, once the beads are loaded to their full capacityof virions, the beads must be cleaned of the virus particles. Resincleaning and lifetime validation costs (while cheaper than purchasingnew chromatographic resin for each separation) are considerable.

There remains a long-felt, and unmet need, for a virus-trapping mediumthat has both high-efficiency and high-capacity for trapping virusparticles, and is also sufficiently low in cost that it can beimplemented as a one-time, disposable medium for removing viralcontamination from biological products.

SUMMARY OF THE INVENTION

In contrast to beads, the entire surface of an adsorptive membrane isavailable for virus binding. At the same time, membranes have a lowcapacity for small impurities. Adsorptive membranes are also cheaper tomanufacture than are controlled-porosity, monodisperse beads. Adsorptivemembranes for viral clearance are sufficiently cheap to allow themembranes to be disposable. Thus, ligand candidates for chromatographicbeads that are rejected due to their irreversible bindingcharacteristics are ideal candidates for viral clearance usingadsorptive membranes.

Thus, the present invention includes using ligands that do not bindmAbs, but do bind virus particles under a range of conductivities and pHvalues.

One distinct advantage of the present invention is that by establishingbracketed generic conditions for viral clearance by disposable membraneadsorbers, developers of new mAbs (and other biologics) will be able tocite project results in lieu of performing costly and time-consumingvalidation studies, thereby freeing resources and accelerating theavailability of therapeutic products to US health-care consumers.

Thus, the present invention is directed to a disposable, virus-trappingmembrane comprising a disposable, micro-porous filter membrane and aligand immobilized on the membrane. The ligand is dimensioned andconfigured to irreversibly and selectively bind viruses andsimultaneously to have a pKa sufficiently high to repel basic proteins(including antibodies in general and monoclonal antibodies inparticular) via electrostatic charge repulsion. It is preferred that theligand is a multi-modal anion-exchange ligand and that the ligand has apositive charge at pH 7. The ligand also preferably has a pKa of atleast 10.0.

The filter membrane itself may be fabricated from any suitable,non-reactive material. Preferably, the membrane is fabricated from apolymeric substrate material, for example a polymer substrate selectedfrom the group consisting of polyvinylidene difluoride,polytetrafluorethylene, polyamides, polyamide-imides, polysultones,polyethersulfones, and polyphenylsulfones.

It is preferred that the ligand is dimensioned and configured to yield alog-reduction value (LRV) of at least 1.0 for neutral viruses disposedin a solution comprising up to 50 mM salt, and more preferably up to 150mM salt. It is more preferred still that the ligand is dimensioned andconfigured to yield a log-reduction value (LRV) of at least 5.0 forneutral viruses disposed in a solution comprising up to 50 mM salt andmore preferably up to 150 mM salt.

The ligand may be selected from the group consisting of tyrosinol,tryptophanol, octopamine, 2-aminobenzimidazole,1,3-diamino-2-hydroxypropane, tris(2-aminoethyl)amine, and agmatine.Tris(2-aminoethyl)amine and agmatine are most preferred.

The invention is also directed to a corresponding method of using themembranes described herein to trap viruses, thereby removing them from asolution. Thus, the invention is also directed to a method of removingviruses from a solution suspected of containing viruses, the methodcomprising contacting a solution suspected of containing viruses withvirus-trapping membrane comprising a disposable, micro-porous filtermembrane and a ligand immobilized on the membrane, wherein the ligand isdimensioned and configured to irreversibly and selectively bind viruses,and has a pKa sufficiently high to repel basic proteins present in thesolution via electrostatic charge repulsion.

As noted earlier with respect to the membrane itself, in the method itis preferred that the solution is contacted with the virus-trappingmembrane for a time sufficient to a yield log-reduction value (LRV) ofat least 1.0 for neutral viruses disposed in the solution when thesolution comprises from 0 to about 50 mM salt, and more preferably stillto a yield log-reduction value (LRV) of at least 1.0 for neutral virusesdisposed in the solution when the solution comprises from 0 to about 150mM salt. It is still more preferred that the solution is contacted withthe virus-trapping membrane for a time sufficient to a yieldlog-reduction value (LRV) of at least 5.0 for neutral viruses disposedin the solution when the solution comprises from 0 to about 50 mM salt,and more preferably still to a yield log-reduction value (LRV) of atleast 5.0 for neutral viruses disposed in the solution when the solutioncomprises from 0 to about 150 mM salt.

The filter membrane preferably comprises polyvinylidene difluoride(PVDF), although it may be fabricated from any non-reactive,micro-porous filter membrane material now known or developed in thefuture. Illustrative filter membrane materials include (withoutlimitation), PVDF (e.g., “KYNAR FLEX”®-brand PVDF, commerciallyavailable from Arkema, Inc., King of Prussia, Pa.),polytetrafluorethylene (PTFE), other fluorinated polymers, polyamides(e.g., nylon), polyamide-imides, polysulfones, polyethersulfones,polyphenylsulfones, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph depicting breakthrough curves predicted using Eqn. 1for various values of the dimensionless number of transfer units “n.”Larger values for “n” yield sharper breakthrough curves.

FIG. 2 is a graph depicting breakthrough curves predicted using Eqn. 5as a function of the fraction unmixed volume (“x”), where τ_(sys) is thesystem mean residence time.

FIG. 3 is a graph depicting experimental breakthrough curves for anon-binding tracer superimposed upon fitted curves using Eqn. 5 aloneand Eqns. 5 and 6 combined.

FIG. 4 is a graph depicting an experimental breakthrough curve foralpha-lactalbumin superimposed upon a fitted curve using Eqn. 1.

FIG. 5 is a graph depicting log reduction value (LRV) as a function ofthroughput for the bacteriophage φX174 in 25 mM Tris, pH 8.1, at a flowrate of 3,400 membrane volumes per hour.

FIG. 6 is a graph depicting the calculated decline in LRV versus thethroughput parameter (T) for an adsorptive membrane for trapping viruseswhen adsorption is reversible (r=1) and irreversible (r=infinity).Irreversible adsorption is desired for virus removal because the LRV isgreater at any given throughput, and the throughput is greater at agiven LRV when adsorption is irreversible rather than reversible.

FIG. 7 is a schematic diagram of multi-modal ligand binding.

FIG. 8 is a graph depicting the log reduction value (Y-axis) of“MUSTANG”® Q-brand membrane filters as a function of throughput (X-axis)and salt concentration (□=0 mM NaCl; ▴=50 mM NaCl; ▪=150 mM NaCl).“MUSTANG” is a registered trademark of the Pall Corporation, East Hills,N.Y. See Example 1 for details.

FIG. 9 is a histogram depicting the capacity of each ligand listed inTable 2 to bind an anionic dye and a negatively-charged protein (bovineserum albumin, BSA). See Example 2 for details.

FIG. 10 is a graph depicting the performance of a virus-trappingmembrane containing the immobilized ligand agmatine to remove a neutralvirus (φX174) from a solution comprising a basic protein (0.5 g/Lribonuclease) and different concentrations of salt (□=0 mM NaCl; ▴=50 mMNaCl; ▪=150 mM NaCl).

DETAILED DESCRIPTION

Most chromatographic separations utilize columns packed with beads. Thebead diameter is an important factor: small beads result in fastdiffusion times and large numbers of theoretical plates, but also highpressure drops. Large beads are used in process scale separations toallow for increased flow rates without incurring high pressure drops andthe resulting bed compression and eventual plugging. However, largebeads have long diffusion times, low plate numbers, and low dynamiccapacities. In 1988, membrane chromatography was first introduced as ameans to overcome the limitations of column chromatography (Brandt etal., 1988). Microporous membranes containing immobilized ligands wereused as the chromatographic media. Because the membranes were thin (˜0.1mm), pressure drop limitations were not significant. Diffusionlimitations were eliminated because solute was transported through thepores of the membrane by convection not diffusion. The first deviceswere hollow fiber membranes where the surface was activated for affinityligand attachment.

Membrane chromatography has evolved since 1988. Several reviews ofmembrane chromatography spell out the evolution of the technology overthe years. (See, e.g., Etzel 2003, Ghosh 2002, Zeng et al. 1999,Charcosset 1998, Roper 1995, Thommes 1995.). Single-layer andhollow-fiber devices were abandoned because of poor performance.Affinity chromatography gave way to ion exchange chromatography as theprimary ligand type. Vendor promotion turned away from proteinpurification to purification of large biomolecules such as plasmid DNA,viruses, and very large proteins (>250 kDa) where chromatography beadshave low capacity. Applications such as viral clearance and purificationof gene therapy vectors are examples. Three primary vendors have emergedfor membrane chromatography products: Millipore Corporation (Bedford,Mass., USA, “INTERCEPT”®-brand products), Pall Biopharmaceuticals (EastHills, N.Y., USA, “MUSTANG”®-brand products), and Sartorius AG(Goettingen, Germany, “SARTOBIND”®-brand products). The principles andexperimental methods applicable to membrane chromatography are presentedherein to provide a more complete disclosure of the present invention.

Two key advantages of membrane chromatography over columns packed withbeads are: (1) mass transfer limitations are reduced or eliminatedleading to fast binding of the solute to the ligand sites on themembrane surface; and (2) low trans-membrane pressure drop. For thetarget solute to be captured by the binding sites on the membranesurface, the solute must flow into the pore structure, diffuse to thewall of the pore, and bind to the ligand. The result of this process isthat the solution passing out of the membrane (the effluent) is lessconcentrated in the solute than is the feed solution. The breakthroughcurve (BTC) is a plot of the solute concentration in the effluentsolution versus either time or effluent volume. Ideally the BTC issharp, meaning no solute comes out in the effluent solution until themembrane reaches saturation, at which point the solute concentration inthe effluent solution is the same as in the feed solution. The extent towhich this is not the case is a measure of the impact of slow adsorptionkinetics, slow mass transfer, and mixing in the flow system. The fasterthe flow rate, the more likely the BTC will be broad. The followingparagraphs present the principles of mass transfer, adsorption kinetics,and mixing in the flow system in the context of describing the sharpnessof the BTC.

A simple algebraic model of the BTC can be derived for the case ofirreversible adsorption in the absence of axial dispersion in themembrane, mass transfer limitations, and mixing in the flow system(Heister & Vermeulen 1952). This model was derived from the continuityequation using Langmuir adsorption kinetics as the constitutiverelation:

$\begin{matrix}{{C = \frac{1}{1 + {( {1 - e^{- n}} )e^{n{({1 - T})}}}}},} & (1)\end{matrix}$where C=c/c₀, c=effluent concentration, c₀=feed solution concentration,n=dimensionless number of transfer units, and T=dimensionlessthroughput. Axial dispersion in the membrane is typically negligible,and irreversible adsorption is often a good approximation for processscale protein purification, because the equilibrium dissociationconstant is small for tight binding, and c₀ is large. Therefore theratio c₀/K_(d) approaches infinity, and adsorption is essentiallyirreversible. The parameter T for irreversible adsorption(c_(O)/K_(d)>>1) is given by the equation:

$\begin{matrix}{{T = {\frac{ɛ\; c_{0}}{( {1 - ɛ} )c_{1}}( {\tau - 1} )}},} & (2)\end{matrix}$where ε is the void fraction of the membrane, and c₁ is the total ligandcapacity of the membrane based on the solid volume of the membrane. Thethroughput parameter is a measure of the loading of the membrane. It isthe ratio between the amount of solute loaded into the membrane via thefeed solution and the maximum amount of solute that can bind to themembrane. The dimensionless time is defined by τ=vt/L, Where v is theinterstitial liquid velocity, L is the membrane thickness, and t istime.

The parameter n (number of transfer units) is given by the equation:

$\begin{matrix}{{n = \frac{( {1 - ɛ} )k_{a}c_{1}L}{ɛ\; v}},} & (3)\end{matrix}$where k_(a) is the association rate constant of the solute with theligand. The parameter L/n is the height of a transfer unit, comparableto the height equivalent to a theoretical plate (HETP) commonly found inthe chromatography literature. When n is large, or HETP is small,breakthrough curves and elution peaks are sharp.

Equation (1) is plotted for various values of n in FIG. 1. The BTC isreasonably sharp when n=20-25. Not much is gained by going to n=50 andbeyond. Increasing n requires a high capacity (c₁), a fast associationrate constant (k_(a)), and a long residence time in the membrane (L/v).If a high flow rate is desired, as is usually the case, then one or moreof the other parameter values must have a large value. Thus, mostchromatographic membranes use ion exchange binding (high k_(a)), a highligand density (high c₁), and several layers (high L) to achieve sharpBTCs at high flow rates (high v).

The assumption of irreversible adsorption made in the derivation of Eqn(1) is valid for values of c₀/K_(a) approaching infinity. The practicalcut-off for when c₀/K_(a) is large enough was determined to bec₀/K_(d)>60, set by the criteria that Eqn (1) fall within 95% of theexact solution at C=0.1 for finite values of c₀/K_(d). In other words,the exact solution for C=0.1 was used to find T, and then the value of Cfrom Eqn (1) at that T had to be within 95% of the exact solution.

To eliminate mass transfer effects, the residence time in the membrane(L/v) must be much greater than the time scale for diffusion from thecenter of the membrane pore to the wall:L/ν>>d _(p) ²/4D  (4)where d_(p) is the diameter of the pore and D is the diffusioncoefficient of the solute. This situation is frequently not the casewhen the membrane is thin (small L), the pores are large (large d_(p)),and operation is at high flow rate (large v). Most membranechromatography systems are operated at residence times of 1 to 10seconds. Membrane pore sizes of less than 1 μm eliminate mass transferlimitations for large proteins when residence times are about 1 secondor longer. However, some membranes have a pore size of about 5 μm, inwhich case residence times of about 100 seconds or longer are requiredto obtain sharp BTCs for large proteins. For very large biomoleculessuch as plasmid DNA and viruses, even longer residence times are neededbecause D is smaller. As a rule of thumb, D is approximatelyproportional to the inverse of the molecular mass raised to the ⅓ power.Therefore, systems separating small proteins such as alpha-lactalbumin(14.4 kDa, D=1.1×10⁻⁶cm²/s) can be operated at higher flow rates thansystems separating large proteins such as thyroglobulin (660 kDa,D=2.5×10⁻⁷ cm²/s).

A few examples illustrate the use of Eqn (4). BTCs were sharp whenalpha-lactalbumin and thyroglobulin were captured onto a chromatographicmembrane having a pore size d_(p)=0.65 μm, a stack thickness L=0.098 cm,and operated at velocity v=4.9×10⁻³ cm/s (Yang et al. 2002).

On one hand, the time scales for diffusion (4 ms for thyroglobulin and 1ms for alpha-lactalbumin) were much smaller than the residence time inthe membrane (L/v=20 s). On the other hand, BTCs were broad whenthyroglobulin was captured onto a chromatographic membrane having a poresize of 5 μm, a stack thickness of 0.06 cm, and operated at a velocityof 4.2×10⁻² cm/s. In this case, the time scale for diffusion (0.25 s)was too close to the residence time in the membrane (L/v=1.4 s). Even ata residence time of 14 seconds the BTC was not sharp for this system,which indicates that the residence time in the membrane needs to be muchgreater than the time scale for diffusion to obtain a sharp BTC.

Broad BTCs can result solely from liquid mixing in the pump, tubing,fittings, membrane holder, stack of membranes, and detector system. Forexample, if the liquid flowing through the membranes has differentresidence times, e.g. shorter times through the center and longer timesthrough the edges, then it will broaden the BTC. The simplest modelfound to describe mixing in the flow system in membrane chromatographyis the serial combination of a continuously stirred tank reactor (CSTR)and an ideal plug flow reactor (PFR) (10):

$\begin{matrix}{{C = {1 - {\exp( \frac{x - ( {\tau\text{/}\tau_{sys}} )}{1 - x} )}}},} & (5)\end{matrix}$

where τ_(sys) is the dimensionless mean residence time in the system,and x is the fraction PFR volume (x=τ_(PFR)/τ_(sys)). After the delaytime=xτ_(sys) from the dead volume, Eqn (5) can be used to predict theBTC for a non-binding tracer. Prior to that time (τ≦xτ_(sys)) C=0 (FIG.2). Typically, mixing in the flow system is not a significant factor indetermining the shape of the BTC because xτ_(sys) is small compared tothe values of τ at the point of breakthrough, defined as when C=0.1.

The following example illustrates how to conduct an experiment andanalyze the results. Data were taken from the literature for capture ofa small protein (alpha-lactalbumin) by an anion exchange membrane (Yanget al. 2002).

Flat-sheet polyvinylidene difluoride membranes (acylimidazole activated“DURAPORE”®-brand membranes, Millipore, Bedford, Mass.) were reactedwith 2-amino-ethyltrimethylammonium chloride to make the anion exchangemembranes. These membranes were 140 μm thick and had a pore size of 0.65μm, an internal surface area of 155 cm² per cm² of frontal area, and avoid fraction of ε=0.7. A 7-layer stack of these 25 mm diametermembranes sandwiched between 2 blank membranes upstream and downstream(11 membrane discs total) was placed into a membrane holder. The blankmembranes aided in flow distribution. Protein solution (0.05 g/Lalpha-lactalbumin in 50 mM Tris, pH 8.3) was loaded into the membranestack at a flow rate of 1 mL/min, and the absorbance at 280 nm of theeffluent solution measured versus time. Mixing in the flow system wasmeasured by loading a non-binding tracer (0.05 g/L alpha-lactalbumin in50 mM Tris, 2 M NaCl, pH 8.3).

The response to loading a non-binding tracer was fit using Eqn (5)resulting in a fraction PFR volume of x=0.67 and a dimensionlessresidence time for the system of τ_(sys)=9.4 (see FIG. 3). To generatethis plot from the raw data, the voltage signal from the detector wasdetermined for the baseline (V_(BL)) using only buffer without protein,and the feed solution (V_(FS)) while bypassing the membrane holder. Thenthe voltage signal from the BTC was converted to C using the equationC=(V−V_(O))/(V_(FS)−V_(O)). This conversion assumes that absorbance islinearly related to protein concentration, which is a valid assumptionfor dilute protein solutions (c<2 g/L) as was the case in this example(c₀=0.05 g/L). The x-axis was obtained by converting time todimensionless time τ(=vt/L) using the values of v=4.85×10⁻³ cm/s (v=Q/εAwhere Q=1 mL/min, ε=0.7, and A=4.91 cm²) and L=0.098 cm (=7×140 μm).

The values of x and τ_(sys) mentioned above were obtained using theSOLVER function in Microsoft Excel software to minimize the sum of thesquare of the difference between the model and the data (least squaresmethod). Another perhaps more accurate method is to obtain τ_(sys), fromthe first moment of the data using the equation:

$\begin{matrix}{\tau_{sys} = {{\int_{0}^{\infty}{( {1 - C} )\ d\;\tau}} = {\int_{0}^{1}{\tau\ d\;{C.}}}}} & (6)\end{matrix}$

Then this calculated value of τ_(sys), is used along with Eqn (5) to fitthe data by using x as the only fitted parameter value in Excel. Usingthis method, τ_(sys)=10.3 and x=0.638. This result is also plotted inFIG. 3 and is nearly identical to the first method.

Frequently, rather than reporting liquid volumes directly, the volumesare normalized by dividing by the membrane volume. This makes theresults dimensionless and independent of scale. The volumes are thenreferred to in terms of “membrane volumes.” For example, to normalizethe effluent liquid volume and express it in terms of membrane volumes,divide it by the membrane volume: (effluent volume)÷(membranevolume)=ετ. When the system volume is normalized and expressed in termsof membrane volumes, it is equal to: ετ_(sys)=7.2 membrane volumes forthese data (Yang et al. 2002). Of this, xετ_(sys), =4.6 membrane volumesis the PFR portion, which includes 1 membrane volume for the stack of 7membranes, and (1−x)ετ_(sys),=2.6 membrane volumes is the CSTR portion.One membrane volume equals 0.481 mL in this experiment. In conclusion,if the value of T at the point of breakthrough (C=0.1) is much greaterthan xτ_(sys)=6.3 to 6.6, then mixing in the flow system can beneglected.

To ignore mass transfer effects, Eqn (4) must be satisfied. For theexperimental system described herein, L/v=20 s, and the RHS of Eqn (4)is 1 ms=[0.65×10⁻⁴ cm)²/4(1.1×10⁻⁶ cm²)]. Therefore, the time scale forconvection in the membrane is 20,000 times greater than the time scalefor boundary layer mass transfer to the wall of the pores, and masstransfer can be safely neglected. Based on this calculation, a greaterflow rate than 1 mL/min, perhaps even 200 mL/min, could have been usedand still not have a mass transfer limitation. Thus, although the flowrate used was 125 membrane volumes per hour, it might have been possibleto use 25,000 membrane volumes per hour without encountering a masstransfer limitation. Column chromatography using beds of packed beadstypically operates at flow rates of 30 column volumes per hour, muchlower than the flow rates possible using membrane chromatography.

The experimental BTC for alpha-lactalbumin is shown in FIG. 3. The pointof breakthrough (C=0.1) occurred at τ=93.6. This value is 14 to 15 timesgreater than xτ_(sys), which means that mixing in the flow system can beneglected as a factor in determining the shape of the BTC. The point ofbreakthrough occurred at 66 membrane volumes (=εt). The dynamic bindingcapacity of the membrane is then ετc₀ or 3.3 mg/mL expressed as mg boundper mL of membrane.

To fit Eqn (1) to the BTC, values of the two unknowns (k₂ and c₁) wereassumed temporarily, allowing calculation of T using Eqn (2) and n usingEqn (3). The other parameter values (ε, c₀, v, L, and τ) are alreadyknown. Using the temporary values of T and N, Eqn (1) was used tocalculate C. Then SOLVER in Excel was used to minimize the square of thedifferences between the calculated and observed values of C using K_(a)and c₁ as fitted parameters. The solution found was k_(a)=1900 M⁻¹s⁻¹and c₁=0.00085 M. The value of n was 14.

The fitted value for c₁=0.00085 M is expressed as moles ofalpha-lactalbumin bound per L of membrane solid volume. The solid volumeof the membrane divided by the total volume of the membrane equals(1−ε). Therefore, the fitted value of the membrane capacity is 3.7 mg/mLwhen expressed on a mass and total-membrane-volume basis (=(1−ε)c₁).This value corresponds closely to the value of 3.3 mg/mL determined fromthe point of breakthrough as mentioned above. In conclusion, the fittedand observed binding capacities match, which provides validation of themodel and the fitted parameter values. The BTC was not symmetric.Instead, the BTC first rose sharply toward C=0.6-0.8, and then roseslowly toward, but never reached C=1.0 (see FIG. 4).

Successful scale-down and scale-up of membrane chromatography systemsrequires an accurate, scientifically based model. Eqns (1)-(6) can beused for this purpose. To obtain equal BTC performance (C vs. time isthe same), the values of n and T must match at each time point for thesmall- and large-scale, and mixing in the flow system (x and τ_(sys))must be either the same or small enough to be negligible. When the samemembrane material and feed stream are used at large- and small-scale,parameters such as c₀, ε, c₁, k_(a) d_(p), and D will most likely beconstant. However, v, L, x, and τ_(sys), may not be constant, becausethe flow rate, number of layers in the membrane stack, and extent ofmixing in the flow system may increase with increasing scale. However,if L/v is kept constant, and mixing in the flow system is verified to benegligible, then equal performance at different scales is expected. Theimpact of potential deviations in operating parameters (c₀ and v), andmembrane chromatography device parameters (ε, c₁, k_(a), d_(p), and L)can then be evaluated using the model, and used to steer clear ofregions where performance is too sensitive to normal variation.

As noted earlier, the potential for contamination of therapeuticproteins produced in cell culture by viruses is a regulatory concern.Steps are included in downstream processing specifically to meetregulatory requirements; redundant and complementary unit operations areincluded that clear any potential viral contaminant from the proteinproduct. For viral clearance applications, performance is measured bythe log reduction value (LRV), which is simply LRV=−Log₁₀(C). TypicalLRV values for anion exchange column chromatography are LRV=4 to 6(Curtis et al. 2003).

The assumption of irreversible adsorption made in the derivation of Eqn(1) is valid for values of c₀/K_(d) approaching infinity, as mentionedabove. This is a good assumption for the BTC in process-scale proteinseparations where the feed solution is concentrated. In contrast, inviral clearance operations, the feed solution typically contains verysmall concentrations of virus (pM to nM). Therefore, depending on thevalue of K_(d), two limiting cases are possible: (1) C_(p)/K_(d)>>1 andirreversible adsorption; and (2) c₀/K_(d)<<1 and linear adsorption.

For irreversible adsorption, where c₀/K_(d) approaches infinity, thepractical cut-off for when c₀/K_(d) is large enough was found to bec₀/K_(d)>30, determined by setting the criteria that Eqn (1) fall within95% of the exact solution at LRV=4. The mathematical relationshipbetween LRV, T, and n for irreversible adsorption can be derived fromEqn (1):

$\begin{matrix}{{LRV} \approx \frac{n( {1 - T} )}{\ln(10)}} & (7)\end{matrix}$

Eqn (7) reveals that there is a linear decline in LRV with increasing T.The slope of this plot is approximately −n/ln(10), and the y-interceptis approximately n/ln(10). See FIG. 6.

For irreversible adsorption, Eqn (2) can be rearranged to find thenumber of membrane volumes processed (ετ) at any value of the parameterT when τ>>1:

$\begin{matrix} {ɛ\tau} \middle| {}_{irreversible}{\approx \frac{{T( {1 - ɛ} )}c_{1}}{c_{0}}}  & (8)\end{matrix}$

The parameter T in Eqn (8) is a dimensionless measure of the relativeamount of material loaded into the membrane. T=0.0 corresponds to thepoint where the feed solution has just started to emerge at the exit ofthe membrane. T=1.0 corresponds to the point where the total mass loadedinto the membrane equals the total membrane capacity. For an infinitelysharp BTC (n→∞), T=1.0 also corresponds to 100% saturation of themembrane. However, this is impractical. A practical target for operationcan be found by examination of Eqn (7). Practically speaking, a LRV=4,in combination with a large loading capacity is suitable for mostapplications. For example, the experiment described herein attainedLRV=4 at T=0.08 and n=10, or at T=0.90 and n=90. Therefore, it isdesirable to have a large value of n because a much larger throughput(greater T) can be achieved, while still attaining LRV=4. To attain 90%of the saturation capacity (T=0.9) at LRV=4.0, Eqn (7) reveals that the90% saturation capacity requires attaining a value of n=92.

From Eqn (3), attaining n=92 requires a high capacity (c₁), thickmembrane stack (L), low flow rate (v), and fast adsorption rate constant(k_(a)). For example, for the membrane system analyzed in section 3, theinvariant membrane parameters are: k_(a)=1900 M⁻¹s⁻¹, c₁=0.00085 M, andε=07. Therefore, to attain the above target (LRV=4 at T=0.9) requiresL/v=133 s. This residence time is much longer than the time used in theexperiment (L/v=20 s). This example illustrates a general rule of thumb:it is easier to obtain a sharp BTC for protein purification than it isto achieve a target LRV for viral clearance.

For the linear adsorption case where c₀/K_(d)<<1, Eqn (1) is not valid.In this case, the BTC is given by:

$\begin{matrix}{C = {1 - {{\exp( {- {nT}} )}{\int_{0}^{n}{{\exp( {- \eta} )}{I_{0}( {{2\sqrt{ {\eta\;{nT}} )}\ d\;\eta},} }}}}}} & (9)\end{matrix}$where I₀ is the modified Bessel function of zero order. Values of n andT that result in LRV=4 were calculated from Eqn (9). In general, whenLRV=4 for any given value of n, the corresponding value of T is smallerin the linear adsorption case than the irreversible adsorption case. Inother words, as in the irreversible adsorption case of Eqn. (7), LRV forthe linear adsorption case is a function of only n and T, but the valuesof LRV for the linear adsorption case are generally smaller at a givenvalue of n and T. Only when T=0 is the LRV for the linear adsorptioncase equal to the LRV for the irreversible adsorption case. This isbecause when T=0, Eqn (9) reduces to C=exp(−n), because I₀(0)=1, andLRV=n/ln(10), which is the same result as Eqn (7) when T=0.

The definition of T is different for the linear adsorption case:

$\begin{matrix}{{T = {\frac{ɛ\; K_{d}}{( {1 - ɛ} )c_{1}}( {\tau - 1} )}},} & (10)\end{matrix}$where K_(d) is the dissociation equilibrium constant Eqn (10) can berearranged to calculate the membrane volumes of feed solution processedat any value of T when τ>>1:

$\begin{matrix} {ɛ\tau} \middle| {}_{linear}{\approx \frac{{T( {1 - ɛ} )}c_{1}}{K_{d}}}  & (11)\end{matrix}$

We can see from Eqn (11) that the volume of feed solution processed at agiven value of T is not related at all to the feed solutionconcentration for the linear case, whereas for the case of irreversibleadsorption it was inversely related to the feed solution concentrationas in Eqn (8). Also, because K_(d)>>c₀ for the linear adsorption case,throughput expressed as ετ or T is going to be lower than for theirreversible adsorption case.

From a regulatory perspective, if a membrane chromatography product wasshown to attain LRV=4 for a particular feed solution at a fixedconcentration (c₀), loading volume (ετ), and residence time (L/v), thenthe LRV should exceed 4 for a smaller loading volume, longer residencetime, or more dilute feed solution. Validation of a membranechromatography system for viral clearance should utilize measuring theLRV of effluent fractions over time rather than the entire effluentpool, and the trend of LRV vs. T can be determined to aid in settingallowable operating limits.

From a membrane design point of view, we have set the above target(LRV=4.0 and T=0.9), but need to set some additional constraints tofully define the problem. For example, what flow rate and volumetricthroughput will be attractive compared to competing technologies? Oneapproach to answering this question is to take values for the flow rateand volumetric throughput from the commercially successful viralfiltration systems. It should be noted that viral filtration removesviruses by a sieving mechanism, which is totally different than theadsorption mechanism used in membrane chromatography. Nevertheless, theperformance capabilities of viral filtration membranes can be used as atarget for membrane chromatography systems too. A commerciallysuccessful viral filtration system is the “VIRESOLVE”®-brand filtersfrom Millipore (Bedford, Mass.). These filters can achieve LRV=4.0 forthe bacteriophage φX174 when operated at a flow rate of 150 L/m²−h, athroughput of 300 L/m², and a pressure drop of 2.0 bar (MilliporeTechnical Brief 2002). This flow rate and throughput target correspondsto εv_(min)=4.2×10⁻³ cm/s and ετ_(min)/L_(min)=30 cm, respectively, fora membrane chromatography system. One advantage of membranechromatography is a lower pressure drop. At 2.0 bar, the membrane systemanalyzed earlier would attain the target flow rate when L=6.2 cm basedon the reported hydraulic permeability (Phillips et al. 2005).Therefore, pressure drop is not a limitation.

Eqns (7) and (8) can be used for the irreversible adsorption case, andEqn (3) to calculate the minimum L under conditions constrained bymeeting the targets for flow rate εv_(min)=4.2×10⁻³ cm/s) throughput(ετ_(min)L_(min)=30 cm), and viral clearance (LRV 4.0) as set above. Thedata from above is used to illustrate these calculations. The value ofL_(min) needed to meet the flow rate and viral clearance targets isfound from substitution of LRV=4.0 and T=0.9 into Eqn (7) to obtainn=92, which is then substituted into Eqn (3) along withεv_(min)=4.2×10⁻³ cm/s to solve for L_(min). The value of L_(min) neededto meet the throughput target is found from substitution ofετ_(min)L_(min)=30 cm into the LHS of Eqn (8). To meet the throughputrequirement, the membrane must have an L_(min)=0.46 cm. However, thisvalue is too thin to meet the viral clearance target of LRV 4.0, whichrequires L_(min)=0.8 cm. Thus, a membrane stack thicker than L_(min)=0.8cm would exceed the targets set above. The principles outlined hereincan be used to guide the design of membrane chromatography systems forviral clearance. Desirable system parameters include: (1) high membranecapacity c₁, (2) thick membrane stack L, (3) dilute feed solution c₀,and (4) fast association rate constant k_(a). This is in the case ofirreversible adsorption.

The solution is slightly different for the case of linear adsorption. Inthat case, must be known, Eqn (11) comes into play, and the feedsolution concentration does not affect performance. Realistically, theabove target (LRV=4 at T=0.9) cannot be obtained in the case of linearadsorption because T<0.9 when LRV 4 for all reasonable values of n.Thus, the throughput T is less at a given value of n and LRV, and theLRV is less at a given value of n and T for the linear adsorption casecompared to the irreversible adsorption case. In the linear adsorptioncase, a value of n may be chosen and the value of T determined whenLRV=4. The value of L to meet the viral clearance target is calculatedfrom Eqn (3), and the value of L_(min) to meet the flow rate target iscalculated from Eqn (11).

The model can be used to analyze data taken from the literature (13),where the LRV was measured for a membrane chromatography system similarto the one described hereinabove. The effect of throughput (=ετ) on LRVfor φX174 is shown in FIG. 5. The feed solution in this experiment wasvery dilute: 1.5×10⁷ pfu/mL (c₀≈1×10⁻¹³ M). The membrane capacity wasreportedly c₁=0.0058 M, measured using tosyl glutamic acid, and L=0.1cm, and ε=0.7. From these values, the parameter T in Eqn (2) can becalculated: T≈4×10⁻¹¹. In essence, T≈0, and LRV=n/ln(10) from Eqn (7).Therefore, the LRV is not a function of throughput T, and this may bewhy no dependency on T is observed in FIG. 5.

As noted earlier, viral clearance is essential in the manufacture ofbiotechnology-derived products such as monoclonal antibodies (mAbs).Monoclonal antibody production is currently the fastest growing segmentof the U.S. biotechnology industry, with 30% annual growth and over $7billion in annual sales in 2004 (Das 2003). Regulatory agenciesworldwide, including the United States Food & Drug Administration (FDA),mandate a demonstration of freedom from viral contaminants before a newbiopharmaceutical product is approved for human use. See Fed. Reg. 63,51074-51084 (1998). Key components of the assurance of virus safetyinclude specific virus removal steps such as filtration, as well assmall-scale studies that measure the clearance capacity and robustnessof the virus-removal protocol. In response to the recent plateau of newdrug approvals, the FDA has identified a list of important researchgoals that can accelerate the critical path of pharmaceuticaldevelopment. The modernization of manufacturing science, includingdeveloping improved viral safety strategies, is high on this list ofgoals.

The nanometer-scale size of virus particles makes separation frombiopharmaceutical process intermediates a challenging manufacturingissue. Virus particles bind only to the surface of traditionalchromatography beads because they are too large to enter the finenetwork of pores (Endres et al. 2003; Yamamoto et al. 1999; Lyddiatt andO'Sullivan 1998). Therefore, the binding capacity of porous beads forvirus particles is much lower than it is for smaller molecules that canaccess the full volume of the beads. This phenomenon causes an oddproblem: the binding capacity of chromatographic beads is much greaterfor small impurities, host-cell proteins, and endotoxins, than it is forthe far larger target, virus particles (Yang et al. 2003).

Virus particles bind only to the surface of a membrane too, in the samefashion as for beads. However, membranes have a much larger availablesurface area than do beads. For example, micro-porous membranes have aninternal surface area of about 1.1 m²/mL (Soltys and Etzel 2000),compared to about 0.11 m²/mL for a column packed with 90 μm diameterbeads. In short, the surface area of the membrane is a full order ofmagnitude great than the beads. Furthermore, the adsorptive capacity ofmembranes increases with increasing size of the adsorbed particlebecause the larger particles form a thicker layer on the membranesurface (Endres et al. 2003). The net effect is that adsorptivemembranes have a relatively high capacity for large nanometer-sizedparticles and a relatively low capacity for small molecules (DePalma2003). This is the exact opposite of the situation for beads. Thus, therelative advantage of using membranes versus beads increasesdramatically as the particle size to be trapped increases. This makesadsorptive membranes well-suited for viral clearance.

Traditional chromatography beads were designed for protein separations,not virus separations (Lyddiatt & O'Sullivan 1998). Adsorptive membraneshave the advantage of low cost, small volume, and disposability. Beads,in contrast, were designed for multiple uses. Process economics thusencourages the recycling of beads, often for hundreds (and sometimes forthousands) of cycles (O'Leary et al. 2001). For bead recycling,regeneration is essential. Therefore the ligands immobilized on thebeads must bind their target reversibly. Resin cleaning and lifetimevalidation costs are considerable for beads. These restrictions areabsent for adsorptive membranes because they are disposable and do notneed to bind the virus reversibly for viral clearance applications.Because reversible binding is not required, irreversible,tighter-binding ligands are practical. The higher dynamic capacity ofadsorptive membranes reduces the adsorbent volume, requiring smallerbuffer volumes, lower consumption of pharmaceutical-grade water, andless floor space for buffer tanks and pumps. These advantages lead toreduced facility costs, a major expense for bioprocessing. The“pass-through-and-dispose” operational mode also reduces the requiredequipment space as compared to chromatography columns, which eliminatesthe need for a dedicated room for this unit operation.

Robust, uniform and predictable viral clearance by membrane adsorbersenables generic and bracketed validation strategies (Anon. 1997). TheFDA defines a generic clearance study as a situation wherein virusremoval and inactivation is demonstrated for several steps in thepurification process of a model antibody. These data may then beextrapolated to other antibodies following the same procedure. Abracketed validation approach is where virus removal/inactivation isdemonstrated for a particular module at two different values of a givenparameter (e.g. ionic strength, dwell time, temperature, etc.) and mayuse any values of that parameter falling within that range. Examples oftwo matrix/bracket studies of robust viral clearance steps (e.g., low pHinactivation and anion exchange chromatography) have been described inthe literature (Brorson et al. 2003; Curtis et al. 2003). Bracketing andgeneric validation of robust virus removal unit operations were proposedby the FDA to streamline and update the overall viral safety assurancestrategy for clinical trial-stage mAbs. These approaches can eliminateredundant testing, impart flexibility during product development, andspur product development.

The invention thus has many advantages. For example, using membraneseliminates labor-intensive column packing and validation, reduces floorspace and equipment requirements, and lowers overall costs due tostreamlined regulatory compliance. Because of their expense, virusremoval validation studies are often a stumbling block for smaller firmsand independent academic investigators performing early phase studieswith novel antibodies. Large biotechnology companies making largequantities of a single or multiple products also benefit greatly fromthe reduction in cleaning and media lifetime validation costs anddecreased use of floor space, buffer components, equipment, tanks andpharmaceutical-grade water.

TABLE 1 Ligand Panel Ligand Aromatic pKa Charge at pH 7 AETMA No >12 +TAEA No 7.7, 10.5 + Tyrosinol Yes 10.8 +

One strong anion-exchange moiety (2-aminoethyltrimethylammoniumchloride, AETMA) and two multi-modal moieties (tris(2-aminoethyl)amine,TARA; and tyrosinol) serve as examples of ligands (see Table 1). Oneadvantage of the present invention is that the membranes exhibit robustviral clearance, even in the presence of relatively large concentrationsof salt (e.g., 150 mM salt). This is important for achieving robustviral clearance because many process solutions used in biopharmaceuticalmanufacture have conductivities in the range of 15-30 mS/cm. Multi-modalligands are anion exchangers with secondary interactions such ashydrogen bonding and hydrophobic interactions that make them more salttolerant (Johansson et al. 2003). Salt tolerance is measured incomparison to the conventional Q ligand (AETMA), which rapidly losescapacity for some viruses (e.g., φX174) at conductivities three- tosix-fold less than the target range, e.g. dropping viral clearance froma six log-reduction value (LRV) to a one (1) LRV in going from 0 to 50mM NaCl (Phillips & Lutz 2003). The Q ligand is a frequent choice foranion exchange chromatography for viral clearance operations (Xu andBrorson 2003; Curtis et al. 2003). The second ligand, TAEA, is anon-aromatic anion-exchange ligand that is positively-charged at pH 7,causing electrostatic repulsion of mAb. TAEA is 25-times more salttolerant than the Q ligand (Johansson et al. 2003).

The third ligand, tyrosinol, is a good example of a ligand rejected foruse in protein purification, but which is highly suitable as a ligandfor a disposable, virus-trapping membrane. Johansson et al. (2003)rejected tyrosinol for protein purification because “results clearlyproved that the aromatic anion-exchangers have too strong secondaryinteractions to be practically useful.” Nevertheless, “aromatic aminesresulted in higher breakthrough capacities compared to the bestnon-aromatic anion-exchangers.” Tyrosinol had a 63% greater dynamicbinding capacity than the average of the five best non-aromaticanion-exchange ligands, and 36 times greater than the Q ligand whentested in high-salt buffer (23 mS/cm). Tyrosinol was rejected byJohansson et al. because it exhibited low recovery of bound protein. Thefundamental goals for a disposable, virus-trapping membrane, however,are different from and diametrically opposed to those in proteinpurification. In virus trapping (as contrasted to protein purification)the binding protein to the filter medium (especially mAb) must beminimized. In virus trapping, the binding of the virus to the filtermedium is preferably irreversible because there is no need to recoverthe bound virus. In contrast, in protein binding, the binding phenomenonmust necessarily be reversible or the desired protein cannot be elutedfrom the column.

Preventing mAb binding can be accomplished by increasing the pKa of theligand so that the mAb and ligand are both charged positive duringloading. This causes electrostatic charge repulsion of the mAb from theligand (Boschetti 2002; Morrow 2004). The virus, in contrast, is eithernegatively charged or is neutral, and binds to the ligand. Tyrosinol,for example, meets these requirements. It has a pKa of 10.8, making itpositively charged at neutral pH. Most therapeutic mAbs tend to havepI's between 8 and 10. This narrow pI range is because therapeutic rnAbstend to be human IgG1, or to a much lesser extent IgG4. Because the Vregions of an antibody are a small percentage of the total molecule,charge is largely determined by isotype. Thus, mAbs are positivelycharged at neutral pH, which prevents their binding to anion exchangemedia (Curtis et al. 2003). Viruses, on the other hand, can have avariety of pI's and many have negative pI's.

The multi-modal ligands for use in the present invention are selectedbased on the above criteria and outcomes, i.e., it is salt tolerant dueto strong secondary interactions and has a high pKa (e.g., >10) causingelectrostatic charge repulsion of the mAb. The ligand is immobilized ona micro-porous membrane and the virus-containing fluid flows through themembrane while the virus is trapped by the ligand. The membranecontaining the bound virus is disposable.

As shown in FIG. 7, the surface of the membrane (M) contains animmobilized moiety that has dual functionality. One of thefunctionalities (X) is charged positive to cause electrostatic chargerepulsion of proteins such as monoclonal antibodies that are chargedpositive at neutral pH. The other functionality (Y) providesnon-electrostatic secondary interactions between the immobilized ligandand the viruses. Both functionalities undergo only non-covalentinteractions with the viruses and proteins. The moiety is immobilized tothe membrane by a linker molecule (L) that by itself may augment one orboth of these functionalities.

For the moiety X⊕, the desired positive charge at neutral pH can beachieved by an amine having a pKa of 7 or greater, preferably a pKa of10 or greater. Primary (RNH₂), secondary (R₂NH), tertiary (R₃N), andquaternary amines (R₄N⁺) are examples.

For the moiety Y, the desired secondary interactions can come fromnon-covalent interactions e.g., Lewis acid-base pairs, hydrogen bonding,hydrophobic interaction, dipole-dipole attraction, induction effects,and dispersion forces. For example, hydrogen-bonding interactionsoriginate from strong dipole-dipole interactions in which a hydrogenatom serves as a bridge between two electronegative atoms, typically F,O, or N. Amides (RC═ONRH) are an example because the hydrogen that iscovalently bound to the N atom can be shared with the O atom on the acylgroup from another amide. Hydrophobic interactions can originate fromnon-polar groups such as alkyl moieties (e.g., methyl, ethyl, propyl,butyl, pentyl, hexyl, heptyl, octyl, isopropyl, isobutyl, and others)and aryl moieties (e.g., phenyl).

For the moiety L, a chain of atoms or molecular subunits can be used tocovalently link the ligand to the membrane. A linker molecule maycomprise an alkyl chain of from 1-20 carbon atoms, a carbohydrate chainof from 1-15 saccharide groups, a dextran chain of from 1-15 saccharidegroups, an amino acids chain of from 1-25 amino acids, glutaraldehyde,polyethylene glycol, diglycidyl ether, and other linker chemistriesknown in the art (Hermanson G T, Krishna Mallia A, and Smith P K,Immobilized Affinity Ligand Techniques, Academic Press, San Diego,1992.)

The membrane may be any microporous membrane material including, but notlimited to, cellulose, regenerated cellulose, cellulose diactetate andtriacetate, cellulose nitrate, polyethylene, polypropylene,polyethersulfone, polvinylidene difluoride, polymethylmethacrylate, andpolycarbonate. The membrane pore size desired is from 0.1 to 10 μm, anda pore size of 0.5 to 1.5 μm is most desired. A membrane with a highsurface area for the internal pore structure is desired, which typicallycorresponds to fine pore sizes. However, if the pore size is too small,then the membrane tends to plug with fine particulates present in thefeed solution. The pore surface chemistry may be modified to includepolymer coatings or brushes to increase capacity over the base materialof the membrane itself. In this case, the ligand chemistry describedabove would be applied to the polymer coating or brushes.

One embodiment of the present invention is to attach separately, but onthe same membrane, the two ligand functionalities shown in FIG. 7. Thatis, ligand X⊕ and Y could be attached separately at sites adjacent toeach other on the membrane surface via separate linker molecules L. Themembrane then has the two desired functionalities: (1) salt tolerancedue to strong non-electrostatic secondary interactions from ligand Y,and (2) electrostatic charge repulsion of proteins such as monoclonalantibodies due to the like charge from ligand X⊕.

A broader impact of the invention disclosed herein is to affectregulatory policy regarding biopharmaceutical manufacture. In short, byapplying chemical engineering principles to regulatory issues of primeconcern to state and federal regulators, the results generated can guidepublic- and health-policy decision-making. A potential major outcome isregulatory relief for academic centers and small biotechnology firmsthat are developing therapeutic mAb products. By establishing bracketedgeneric conditions for virus removal by membrane adsorbers, developersof new mAbs will be able to cite project results in lieu of performingcostly and time-consuming validation studies. Resources will be freedfor more fruitful purposes, accelerating availability of therapeuticmAbs to consumers.

EXAMPLES

The following examples are included solely to provide a more completedescription of the invention disclosed and claimed herein. The examplesdo not limit the scope of the invention in any fashion.

Example 1

A long-standing and unresolved problem exists in the field of viralclearance: conventional adsorptive membrane products for viral clearancecannot remove neutral viruses from feed solutions having even moderatesalt concentrations. Salt tolerance is a critical factor in designingrobust viral clearance technologies because many process solutions usedin the fabrication of biopharmaceutical products must contain salt tominimize product aggregation.

Conventional adsorptive membrane products utilize a quaternary amineligand for binding viruses. The “MUSTANG”® Q-brand product is a goodexample of a conventional filtration membrane that will very readilyremove neutral viruses in solutions that have very small saltconcentrations. By way of illustration, “MUSTANG”® Q-brand membrane willremove a neutral virus (φX174, pI=6.6) in a feed solution at pH 7.5, toa seven (7) Log Reduction Value (LRV), but only if the solution does notcontain any added salt (NaCl). See FIG. 8 (—□—). But performing the sameseparation, using the same feed solution that contains 50 mM NaCl dropsthe LRV capacity of the membrane to essentially zero. See FIG. 8 (—▴—).The same effect is seen at 150 mM NaCl. See FIG. 8 (—▪—). In short, atlow salt concentrations the virus passes freely through the “MUSTANG”®Q-brand adsorptive membrane into the product. Similar results have beenshown by others. For example, when the “MUSTANG”® Q-brand adsorptiveligand was immobilized to a polyvinylidene diflouride (PVDF) membranefrom Millipore, viral clearance of φX174 dropped from 6 LRV to less than1 LRV when salt concentration was increased from 0 to 50 mM salt(Phillips & Lutz 2003). The significance of these results is that saltconcentrations in the 50 mM to 150 mM range are not uncommon in thebiopharmaceutical processing industry.

In these examples, the neutral virus φX174 is used because is one offive model non-pathogenic bacteriophage viruses commonly used infiltration studies to represent the gamut of physical properties ofpotential adventitious viruses of danger to humans (e.g., size, pI,presence or absence of a membrane). In short, the φX174 virus is anart-recognized virus for modeling viral clearance of dangerouspathogens.

The current viral safety assurance strategy for biopharmaceuticalsshould preclude the presence of viruses in the feed solution. Carefulcontrol of cell lines and raw materials should also prevent introducingvirus into the manufacturing process. See, for example, the officialdocument titled “Guidance for Industry: Q5A—Viral Safety Evaluation ofBiotechnology Products Derived from Cell lines of Human or AnimalOrigin,” published September 1998 under the aegis of the InternationalConference on Harmonisation of Technical Requirements for Registrationof Pharmaceuticals for Human Use (ICH). (This document is widelyreferenced in the industry as “ICH Q5A.”) However, despite bestpractices throughout the manufacturing process, viral contaminations inbioprocessing are essentially stochastic events that can arise at anypoint in the process (for example, from contaminated raw materials tocontaminated packaging). Thus, it is impossible to predict with anyaccuracy which virus could be next introduced into a manufacturingprocess. Therefore, a manufacturer must be prepared to remove allviruses, even neutral viruses.

Example 2

To solve the problem of salt intolerance explained in Example 1, eight(8) ligands were examined (Table 2). These ligands were evaluated usingtwo different function tests. In the first function test, bovine serumalbumin (BSA) in 20 mM piperazine, pH 6.0, with and without added NaCl,was incubated with a functionalized regenerated cellulose membrane.Bound BSA was then measured using the bicinchoninic acid (BCA)colorimetric method. Function test 1 measured the static protein bindingcapacity of the membranes under conditions where the pH was close to theisoelectric point of the protein (pI of BSA=5.1). In this function test,the protein had only a weak charge and was sensitive to added salt. Thisfirst function test was designed to mimic the binding of a neutral viruswhere the charge is weak.

Function test 2 measured the LRV for the neutral virus φX174 using thefunctionalized membranes in flow mode with TE buffer (10 mM Tris, 1 mMEDTA, pH 7.5) with and without added NaCl. Each ligand was immobilizedonto regenerated cellulose membranes having 0.45 μm pore diameter usingallyl glycidyl ether. The allylated membrane was then brominated. Lastlythe ligand was coupled to the membrane via the primary amine on theligand. The results are shown in Table 2.

TABLE 2 LIGAND PANEL NaCl BSA Static φX174 Ligand Structure (mM)Capacity (mg/m²) LRV 2-aminoethyltrimethylammonium chloride (AETMA)

 0  50 150 551  42  10 6.6 0.1 0.1 Tyrosinol (TYR)

 0  50 150 638 284 137 6.4 2.7 0.9 Tryptophanol (TRP)

 0  50 150 668 297 131 5.4 1.1 1.0 Octopamine (OCT)

 0  50 150 280 290 250 5.4 1.1 0.7 2-aminobenzimidazole (ABI)

 0  50 150 414 204 108 5.4 1.8 1.0 Phenylalaninol (PHA)

 0  50 150 330  46  10 2.8 0.1 0.0 1,3-diamino-2-hydroxypropane (DHP)

 0  50 150 946 392  85 5.4 2.1 0.1 tris(2-aminoethyl)amine (TAEA)

 0  50 150 929 402 136 7.6 5.8 5.1 Agmatine (AGM

 0  50 150 1000  319 116 5.5 5.5 5.9 Blank  0  19 0.1  50  26 0.1 150 19 0.1

The first ligand in Table 2 is the traditional strong anion-exchangemoiety [2-aminoethyltrimethylammonium chloride, AETMA] used in existingproducts such as the “MUSTANG”® Q-brand membrane. As expected fromExample 1, AETMA was not salt tolerant. It rapidly lost capacity for thevirus (φ174) even at moderate salt concentration.

The other ligands are examples of salt-tolerant moieties taken from thework of Johansson et at (2003) that targeted protein purification (ascontrasted to viral clearance) using agarose chromatography beads. TheJohansson et al. group developed salt tolerant ligands for purifyingproteins using regenerable and reusable agarose beads. The goal of thepresent work, however, is distinct because the problem to be solved isviral clearance in the context of a manufacturing process. The processneeds to be easy, reproducible, and validatable. Thus, in the presentinvention, it is preferred to use disposable membranes. It is alsopreferred that the binding interaction between the virus and themembrane is irreversible. Lastly, in the present invention, the virus iscleared from the process solution selectively; the membrane does notbind the therapeutic protein. In other words, in the present invention,virus is trapped selectively and irreversibly onto the membrane, whilethe therapeutic protein target is not adsorbed to the membrane. Thespent membrane is then disposed to avoid contamination of subsequentbatches of the therapeutic protein product. That is, rather than use apositively-charged ligand to bind the therapeutic protein, as inJohansson et al., the present invention utilizes a diametrically opposedapproach: the positive charge on the ligand is used to repel thepositively charged therapeutic protein, and simultaneously bind thenegatively charged virus, thereby removing it from the processingstream.

As seen in FIG. 9, the capacity of each ligand to bind an anionic dyecorrelated well with the capacity of each ligand to bind anegatively-charged protein (BSA). However, viral clearance for eachligand was not as well predicted by the dye capacity or the proteincapacity (Table 2). In addition, salt tolerance of the protein bindingcapacity was not correlated well with salt tolerance for viralclearance. In short, the results shown in FIG. 9 are relevant becausethey demonstrate that salt-tolerant viral binding characteristics of amembrane are not predictable from the protein-binding capacity of thesame membrane.

For example, the non-aromatic ligands TAEA, DHP, and AGM all had similarcapacities for the dye and for BSA. See FIG. 9. But viral clearance forTAEA was much greater than for DHP or AGM (see Table 2). Moreover, salttolerance of the BSA capacity was similar for TAEA, DHP, and AGM, butsalt tolerance of viral clearance was much less for DHP than for TAEA orAGM. For the aromatic ligands, salt tolerance of the BSA capacity wassimilar for TYR, TRP, and ABI, and highest for OCT (see Table 2). Incontrast, salt tolerance of viral clearance was highest for TYR, andsimilar for OCT, TRP, and ABI. Further still, OCT had a higher salttolerance for BSA than AGM or TAEA, yet OCT gave a substantially lowersalt tolerance for viral clearance than AGM or TAEA. The ability to binda neutral protein and an anionic dye was not predictive for the abilityto bind a neutral virus.

The aromatic ligand TYR is a good example a ligand rejected for use inprotein purification because binding of protein was too strong. The TYRligand, however, is very well suited for use in viral clearance. Asshown in Table 2, in the absence of added salt, TYR exhibited a LRV of6.4 for virus. Clearance dropped to 2.7 LRV in the presence of 50 mMsalt, and 0.9 LRV in the presence of 150 mM salt. These results weresuperior to the AETMA, where clearance was negligible in the presence ofsalt (LRV≈0.1). These results illustrate the point that the requirementsfor salt tolerant and disposable membranes for viral clearance are sodifferent from the requirements for salt tolerant and reusable beads forprotein purification that ligands rejected for the protein purificationare desired and superior for viral clearance.

Example 3

In addition to salt-tolerant viral clearance, another advantage of thepresent invention is that basic proteins, such as monoclonal antibodies(mAbs) are not bound to the membrane. This is accomplished by increasingthe pKa of the ligand so that the mAb and ligand are bothpositively-charged during loading. This causes an electrostatic chargerepulsion of the mAb from the ligand, yet has no adverse effect on theability of the membrane to bind virus. Most therapeutic mAbs have pI'sbetween about 8 and about 10. This narrow pIE range exists becausetherapeutic mAbs tend to be human IgG1, or to a much lesser extent IgG2and IgG4. Because the V regions of an antibody are a small percentage ofthe total molecule, charge is determined largely by isotype. Thus, mAbsare generally charged positive at neutral pH, which prevents binding toanion exchange media (Curtis et al. 2003). Viruses, on the other hand,have a variety of lower pI's, and many are negatively charged at neutralpH.

AGM, for example, meets these requirements. It has a positive charge atneutral pH because the guanidine moiety has a pKa of 12.5. According tothe methods of Example 2, a disposable, micro-porous,regenerated-cellulose virus-trapping membrane was fabricated containingthe ligand AGM immobilized on the membrane. The basic proteinribonuclease A (pI˜9.5) was used as a mAb surrogate. A feed solutioncomprising the neutral virus φX174 and 0.5 g/L of the basic proteinribonuclease A, both dissolved in TE buffer (10 mM Tris, 1 mM EDTA, pH7.5), with and without added NaCl, was loaded into the membrane. Theresults are shown in FIG. 10. Viral clearance was not reduced by thepresence of the basic protein. For example, compared to a viralclearance of 5.5 to 6.0 LRV without the basic protein and without andwith added salt (Table 2), the viral clearance for AGM was not reducedat all after adding the basic protein (LRV≈6.0, FIG. 10). Thesignificance of this example is two-fold: it demonstrates salt-tolerantvirus trapping of the present invention and simultaneous repulsion ofbasic proteins by electrostatic charge repulsion.

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What is claimed is:
 1. A method of removing viruses from a solutionsuspected of containing viruses, the method comprising: contacting asolution suspected of containing viruses with a virus-trapping membrane,wherein the solution comprises a monoclonal antibody, wherein thevirus-trapping membrane comprises a disposable, micro-porous filtermembrane and a ligand immobilized on the membrane, wherein the ligand istyrosinol.
 2. The method of claim 1, wherein the filter membranecomprises a polymer substrate selected from the group consisting ofpolyvinylidene difluoride, polytetrafluorethylene, polyamides,polyamide-imides, polysulfones, polyethersulfones, andpolyphenylsulfones.
 3. The method of claim 2, wherein the filtermembrane has a membrane pore size of from 0.1 μm to 10 μm.
 4. The methodof claim 3, wherein the solution comprises from 50 mM to 150 mM salt. 5.The method of claim 2, wherein the solution comprises from 50 mM to 150mM salt.
 6. The method of claim 1, wherein the filter membrane has amembrane pore size of from 0.1 μm to 10 μm.
 7. The method of claim 6,wherein the solution comprises from 50 mM to 150 mM salt.
 8. The methodof claim 1, wherein the solution comprises from 50 mM to 150 mM salt. 9.The method of claim 1, wherein the filter membrane has a membrane poresize of from 0.5 μm to 1.5 μm.
 10. The method of claim 9, wherein thefilter membrane comprises a polymer substrate selected from the groupconsisting of polyvinylidene difluoride, polytetrafluorethylene,polyamides, polyamide-imides, polysulfones, polyethersulfones, andpolyphenylsulfones.
 11. The method of claim 10, wherein the solutioncomprises from 50 mM to 150 mM salt.
 12. The method of claim 9, whereinthe solution comprises from 50 mM to 150 mM salt.